Conventionaly, in a helical scanning type magnetic recording and reproducing apparatus (hereinafter referred to as "VTR"), especially in a VTR for domestic use, a method for recording one field video signal in one track has been utilized. By way of attempting the miniaturization of the VTR, a system has been devised in which the number of rotations of the rotation drum on which heads are installed is made twice and the diameter of the rotation drum is reduced to a half on an assumption that the relative speed of the head and the magnetic tape is not changed, that is, the characteristics of recording and reproducing is not changed. In this system, one field video of the signal to be recorded is divided into two tracks, and accordingly there exist two signal junctions in a field. When one of the signal junctions is placed at the horizontal blanking period similarly as in the conventional one field-one track recording system, the other signal junction comes at the center of the screen. Accordingly, in order to realize this system it is necessary to obtain a good junction of video signals at the center of the screen, but this is not so simple as will be described below.
Now suppose that an upper half portion and a lower half portion of the screen are recorded and reproduced by two heads, respectively. When there is a difference in tape tension between at the time of recording and at the time of reproduction, there arises a time discontinuation in the head switching position, producing what is called skew. This will be described with reference to FIGS. 4(a)-4(e).
In FIG. 4(a), the reference numeral 2 designates a magnetic tape, the numerals 3a and 3b designate tracks recorded by the heads 1a and 1b, respectively. If the head 1b is positioned at the position B where the recording is conducted at the same time as at the position A when the head 1a comes to the head switching point A at the reproduction, the junction of the signals at the center of the screen is made in a proper manner. This situation is shown in FIG. 4(b). The reference character P in the drawing represents a signal junction on the screen. However, when the position B is deviated to the position B1 caused by the variation in the tape tension the signal between the position B and B1 will be dropped as shown in FIG. 4(c). Furthermore, when the position B is deviated to the position B2, the signal between the position B2 and B will be repeatedly reproduced as shown in FIG. 4(d). Herein, FIGS. 4(c) and 4(d) shows the amount of skew which is exaggerated for purposes of explanation. In an actual VTR the amount of skew is less than a horizontal scanning period, and when it is received by a television receiver for domestic use, it appears as a curve on the screen by an AFC circuit having an internal synchronization system as shown in FIG. 4(e). In any event, when a skew occurs, a signal junction at the center of the screen is visible and undesirable, when viewed.
However, it is quite difficult to eliminate skew in a helical scanning type VTR. This has been an obstacle in the development of a field division type VTR.
Recently, as a result of the increase in the integration and the decrease in the cost of integrated circuits as a result of advances in semiconductor technology, it has become possible to correct the skew or jitter by including a digital time axis correction circuit (this is implemented by a time axis variation detection circuit and a time axis variation correction circuit, and this is hereinafter referred to as "DTBC") with a VTR. For example, in a DTBC shown in Japanese Laid Open Patent Publication No. Sho. 58-124385, the detection of the time axis variation (time axis error) of the input video signal is conducted by detecting a difference between a sampled phase and a predetermined phase, which sampled phase is obtained by sampling the input video signal by a standard clock signal based on a predetermined phase relationship existing between a predetermined system television video signal and a local standard clock signal. This correction is conducted in such a manner that the time axis error of the input video signal is divided into a large error component with a length of one sampling clock period as a unit and a small error component which is smaller than a sampling clock period, and the former is corrected by adjusting the timing of the reading out from the shift register properly, and a high preciseness correction of the latter is conducted by replacing the driving clock signals for the D/A converter for taking out a corrected output video signal or for the input side A/D converter by a signal which is obtained by applying a phase modulation to a clock signal locked with a standard synchronous signal in accordance with the time axis error.
However, if recording and reproduction are conducted simply in such a device, signal drops or duplication of signals may arise caused by the skew as described with reference to FIGS. 4(a)-4(e). Especially, when a signal drop arises it is impossible to correct the same even by the DTBC. A well known method to solve this problem will be described below.
FIGS. 5(a) and 5(b) are diagrams for exemplifying the principle of this well known method. As shown in FIG. 5(a), at the recording end the video signals to be recorded of the upper half and the lower half of the screen are time axis compressed in each track, and at the reproduction end the signals are time extended, i.e. expanded, to restore to the original state. FIG. 5(b) shows a track pattern on the magnetic tape 2. The time axis compressed video signals are recorded at the diagonal line sections E of the tracks 4(a) and 4(b). The reference characters C and D designate the head switching points. Accordingly, signal drops or signal duplications can be avoided by properly establishing a compression rate at the recording properly such that the points C, D and the diagonal line sections E do not cross each other due to the skew.
In this conventional method, however, a large capacity memory of about a half field is required and two different kinds of clock frequencies are required, whereby the circuit size is undesirably increased.